The invention relates to an electrostatic-shutter image tube comprising:
an electron source for emitting a flow of electrons in response to an incident photon image, the flow of electrons being propagated in the tube through a network of equipotential lines, PA1 means for focusing and accelerating said electrons, PA1 shutter means, situated between the electron source and the focusing and accelerating means, for interrupting the electron flow, and PA1 a target on which the electron flow inscribes a final image.
The invention also relates to an image-pick-up device comprising such an image tube.
When picking-up images, it must generally be possible to interrupt or effect the image pick up in a very short time. Such a mode of operation is determined either by a need to form a rapid sequence of pictures or by a characteristic of the target.
This is, for example, the object of United States Patent Document U.S. Pat. No. 4,528,447 in which a description is given of an electrostatic shutter tube comprising orthogonal pairs of deflection plates. A shutter electrode G1 is manufactured in the form of a cylinder which is provided with a grid. The shutter electrode is disposed between a photocathode and a focusing electrode G2. The photocathode, which receives the incident image to be detected, has a curved surface. The length of the cylindrical shutter electrode G1 is approximately equal to its radius of curvature. Thin, regularly-spaced metal wires assume the shape of a segment of sphere, which is substantially identical to that formed by the concave photocathode, and extend opposite the photocathode.
Such a grid is difficult to manufacture.
When the shutter is "open" during operation, electric fields E.sub.1 and E.sub.2 on both sides of the slits defined by the spaces between the wires must be equal in order not to produce unidirectional microlenses which would have a substantial negative influence on the quality of the final image. The condition E.sub.1 =E.sub.2 can be fulfilled by adjusting the potential V.sub.G1 of the electrode G1 because, in practice, the potential V.sub.G2 of the electrode G2 is determined by the focusing conditions. The potential of G1, which is positive with respect to the photocathode in the open mode of operation, becomes slightly negative to drive back the photoelectrons emitted at a certain initial speed in the closed mode of operation. The electrode G2 is not involved in the shutter operation, because it is situated at a substantial distance from the photocathode and is shielded by the relatively large electrode G1, so that the potential of G2 need not be modulated to improve the shuttering sensitivity.
Moreover, the presence of grid wires may cause problems regarding the manufacture of the photocathode when the latter is manufactured "in situ", because the grid slightly masks the evaporation of the constituents Sn, Cs, K which serve to form the photocathode. Besides, stray emissions may occur at the wires during the operation of the tube.